The present invention relates to a circular arc illumination apparatus such as a reflection type projection aligner, and more particularly to a circular arc illumination apparatus preferable for exposing a large substrate such as a liquid crystal display device having a large screen.
FIG. 7 shows a construction of the projection optical system of a conventional 1:1 reflection type projection aligner comprising a concave mirror 1 and a convex mirror 2. A mask 3 is irradiated by circular arc slit illumination light 4 so as to project the pattern of the mask 3 on a substrate 5. The mask 3 and the substrate 5 are scanned in a direction shown by an arrow B and the direction opposite thereto. The circular arc slit illumination light is generated by enlarging the image of a circular arc lamp or making a point light source by using a reflecting mirror of the rotation symmetry type. The latter method is effective in using far ultraviolet rays. This kind of optical system is disclosed in Japanese Laid-Open Patent Publication Nos. 54-123877 and 57-200012. The apparatus disclosed in the former publication has a spherical mirror, and the apparatus disclosed in the latter publication has a four-dimensional curved surface mirror formed by rotating an ellipse mirror. FIG. 8 shows the construction of a circular arc illumination apparatus for obtaining circular arc illumination. The apparatus comprises reflecting mirrors 10, 11, and 12 serving as a spherical mirror and a four-dimensional curved surface mirror; a point light source 7; a slit 8; a spherical reflecting mirror 9; and a pin hole 13.
The conventional apparatuses having the abovedescribed constructions have, however, the following three disadvantages: The first disadvantage is illumination nonuniformity. FIG. 9 shows discharge electrodes 14 and 15 of the point light source 7. The emission of the point light source 7 is generated by an arc 7a disposed between the electrodes 14 and 15. The discharge position on the surface of the electrode 15 changes with the elapse of time. As a result, the position of the spot light source is nonuniform. Accordingly, the image of the point light source 7 formed via the reflecting mirror 10 shown in FIG. 8 is varied, which causes the generation of nonuniform illumination. The second disadvantage is inefficiency in the utilization of light because a luminous flux is shaped through the circular arc slit. The third disadvantage is cost and difficulty in manufacture. In the exposure of a large liquid crystal substrate, on which researches and development has been made in recent years, there are demands for the provision of large optical instruments, such as a large reflecting mirror which produces a large quantity of circular arc illumination light. Therefore, the cost for manufacturing the apparatus is inevitably high, and further, it is difficult to manufacture it.
In order to eliminate the above-described disadvantages, the present applicant has proposed a circular arc illumination apparatus comprising a rotary illumination optical system disclosed in Japanese Laid-Open Patent Publication No. 63-102228. FIG. 10 shows the construction of the circular arc illumination apparatus comprising a light source 16; an arc 17 disposed between the electrodes of the light source 16; a spherical reflecting mirror 18; auxiliary condenser lenses 19a and 19b; illumination field stops 20a and 20b; plane mirrors 21a and 21b of the heat ray transparent type; aperture-stops 22a and 22b; and condenser lenses 23a and 23b. The position relationship among the above-described optical instruments is as follows. The image of the arc 17 is formed at the aperture diaphragm 22a via the auxiliary condenser lens 19a. The front focal point of the condenser lens 23a is at the aperture diaphragm 22a. The image of the illumination field stops 20a is formed on the lower surface of the mask 24, thus constituting a Koehler illumination optical system. The optical instruments 19a through 23a are installed on a rotary frame 27, rotatably supported by bearings 28 and connected with a motor 29 via pulleys 30 and 31 and a belt 32. The light source 16 and the spherical reflecting mirror 18 are fixed and thus nonrotatable. In the construction described above, spot illumination having an optical axis parallel with the rotary shaft scans an object on the periphery of the rotary shaft, thus performing a circular arc illumination. Since a great number of large concave mirrors is not used, the apparatus can be manufactured easily and at a low cost.
In the construction disclosed in Japanese Laid-Open Patent Publication No. 63-102228, the mask 24 is not uniformly exposed. FIG. 11 shows the locus of spot light. A circular arc region 33 of the mask 24 is illuminated by spot lights which overlap each other. Supposing that spot light having a diameter of (D) generates a circular arc, the radius of which is (R), a slight region (dxdy) on the mask 24 moves a distance of A-A' at the center of scanning, i.e. the center of the mask, while the slight region (dxdy) moves a distance C-C' at an end of scanning, i.e. an end of the mask 24. Therefore, the time period in which the slight region at the center of the mask 24 is irradiated by the spot lights is different from the time period in which the slight region at the end thereof is irradiated by the spot lights. Thus, there is a difference in the exposure time period between the two slight regions. In this manner, the mask is nonuniformly exposed.